Here is why it is necessary that you know the soil pH, the nitrogen, phosphorus and potash content in your soil.
Soil pH is the factor which determines whether or not plants are able to consume nutrients. If the pH is too high or too low, nutrients in the soil "lock up", become unabsorbable by the plants, thus fertilizer not only goes to waste, but your plants literally starve to death.
Adequate nitrogen produces luxuriant growth of stalks, stems, leaves, and grasses. Excessive nitrogen causes too rapid growth that results in softness of tissue and general plant weakness. Plants suffering from nitrogen deficiency are more susceptible to disease, infection, and injury. Plants given too much nitrogen resume active vegetable ("green") growth which retards flower and seed formation.
Phosphorus gives plants a rapid start, stimulates root formation, hastens maturation, aids blooming and seed formation.
Proper amounts of potash stimulate early root or tuber formation which is essential for all underground vegetables and tuberous flowers. Excessive potash reduces a plant's resistance to droughts and frost injury and delays plant maturity.
HOW TO GATHER SOIL SAMPLE
Gather a soil sample from two to three inches below the surface using a clean instrument such as a soil sampler, trowel or spoon. Since test results are sensitive to external factors such as ashes, never smoke while gathering or testing your soil and avoid touching the sample with your hands.
Put samples in clean containers and label according to which plot they were taken from samples should be taken from various areas, especially when there is a change in the elevation of the land or where there might be a variation in the soil (a particularly sunny spot, an area beneath a tree, a part of your garden that has been under cultivation, or low-lying areas which collect water). It is best to take samples from each corner and the center of the plot in any case.